2025A&A...698A.105Y

Context. Variability is a ubiquitous feature of active galactic nuclei (AGNs), and the characterisation of this variability is crucial to constraining its physical mechanism and proper applications in AGN studies. The advent of all-sky and high-cadence optical surveys allows more accurate measurements of AGN variability down to short timescales as well as direct comparisons with X-ray variability from the same sample of sources.
Aims. We aim to analyse the optical power spectral density (PSD) of AGNs with measured X-ray PSDs.
Methods. We used light curves from the All-Sky Automated Survey for SuperNovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite (TESS) and used the Lomb-Scargle periodogram to obtain PSDs. The joint optical PSD is measured over up to six orders of magnitude in frequency space on timescales of minutes to a decade. We fitted either a damped random walk (DRW) or a broken power law (BPL) model to constrain the PSD model and break frequency.
Results. We find a set of break frequencies (≲10^–2 day^–1) from DRW and BPL fits that generally confirm previously reported correlations between break frequencies and the black hole mass. In addition, we find a second set of break frequencies at higher frequencies (> 10^–2 day^–1). We observe a potential weak correlation between the high-frequency breaks with the X-ray break frequencies and the black hole mass. We further explored the dependence of the correlations on other AGN parameters, finding that adding X-ray, optical, or bolometric luminosity as the third correlation parameter can substantially improve the correlation significances. The newly identified high-frequency optical breaks can constrain different aspects of the physics of AGNs.